Mobile communication has become a necessity in the life of modern people. CDMA is one of the most successful digital wireless technologies that have been developed and employed to this end in many countries around the world. The reasons for the success of CDMA include that it is an advanced digital technology that can offer about 7 to 10 times the capacity of analog technologies and about 6 times the capacity of other digital technologies, such as Time Division Multiple Access (TDMA). The speech quality provided by CDMA systems is superior to any other digital cellular technology, particularly in difficult radio environments, such as dense urban areas and mountainous regions.
In a CDMA system, all users share the time and frequency domains as a base station simultaneously transmits distinct information signals to multiple subscriber mobile stations over a single frequency band. In particular, prior to transmission, the base station multiplies the individual information signal intended for each of the mobile stations by a unique signature sequence, referred to as a pseudo-noise (PN) sequence. This PN sequence can be formed by multiplying a long pseudo-noise sequence with a time offset, which is used to differentiate the various base stations in the network, together with a short code unique to each mobile station. Typically, the long code sequence is generated by a shift register, while the short code sequence can be chosen as an orthogonal code, such as the Walsh codes. The use of such orthogonal codes and their corresponding benefits are outlined in U.S. Pat. Nos. 5,103,459 and 5,193,094, which are incorporated herein by reference.
The multiplication of the information signal by the signature sequence spreads the spectrum of the signal by increasing the rate of transmission from the bit rate of the information-carrying signal to what is known as the chip rate. This effect is known as the spreading the spectrum of the information signal, hence the term spread spectrum communication. Spread spectrum signals from the mobile users are accumulated and then transmitted simultaneously by the base station.
Upon receipt, each mobile station de-spreads the received spread spectrum signal by multiplying the received signal by the mobile station's assigned unique signature sequence. The result is then integrated to isolate the information signal intended for the particular mobile station from the other signals intended for other mobile stations. This way, the previously-spread information sequence of that particular mobile user is mapped back to the original information sequence, while information sequences corresponding to the other users remain occupying a large spectral band and appear as noise. The structure and operation of CDMA systems are well known. See, e.g., Andrew J. Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison-Wesley Publishing, 1995; Marvin K. Simon, Jim K. Omura, Robert A. Scholtz, and Barry K. Levitt, Spread Spectrum Communications Handbook, McGraw-Hill, Inc., 1994, incorporated herein for background.
One of the major advantages of CDMA systems over other multiple-access telecommunications systems is the ability of CDMA systems to exploit path diversity of the incoming radio-frequency (RF) signal. The CDMA signal is communicated from a transmitter to a receiver via a channel including several independent paths, referred to as “multipaths”, illustrated in FIG. 1. Each multipath represents a distinct route that the information signal takes between the transmitter and receiver. The transmitted signal thus appears at the receiver as a plurality of multipath signals or “multipaths”. Each multipath may arrive at the receiver with an arbitrary timing delay, and may have a different signal strength at any time due to signal fading. In a typical communications system the diversity of multipath signals may lead to significant signal interference.
By means of background, consider the multipaths shown in FIG. 1. In the case of a non-stationary transmitter or receiver, in addition to different signal strength and timing delay, each received multipath signal may have a different Doppler shift. The Doppler shift in a signal is known as a change in the frequency of an electromagnetic wave when transmitter or receiver are in motion relative to each other. FIGS. 2A and 2B illustrate the effect of the multipath channel on a short segment of a pilot signal transmitted in a hilly terrain, while FIGS. 3A and 3B illustrate the same effect for an urban environment. In particular, FIG. 2A and FIG. 3A illustrate short segments of the inphase and the quadrature components of the transmitted noise-free pilot channel signal; FIGS. 2B and 3B illustrate the corresponding inphase and quadrature components of the received signal.
The propagation parametes of the signals in FIGS. 2A,B and 3A,B are given in Tables 1 and 2, respectively, which are taken from “3rd Generation Partnership Project,” Technical Specification Group Radio Access Networks, Deployment aspects (Release 4), 3GPP TR 25.943 V4.0.0, June 2001, incorporated herein for background.
TABLE 1Channel for hilly terrain areaTap numberRelative time (μs)average relative power (dB)10−3.620.356−8.930.441−10.240.528−11.550.546−11.860.609−12.770.625−13.080.842−16.290.916−17.3100.941−17.71115.000−17.61216.172−22.71316.492−24.11416.876−25.81516.882−25.81616.978−26.21717.615−29.01817.827−29.91917.849−30.02018.016−30.7
TABLE 2Channel for urban areaTap numberRelative time (μs)average relative power (dB)10−5.720.217−7.630.512−10.140.514−10.250.517−10.260.674−11.570.882−13.481.230−16.391.287−16.9101.311−17.1111.349−17.4121.533−19.0131.535−19.0141.622−19.8151.818−21.5161.836−21.6171.884−22.1181.943−22.6192.048−23.5202.140−24.3
As illustrated in FIGS. 2A, B and 3A, B, although there is no interference or noise the received pilot signals differ considerably from the transmitted ones. Note that for better visualization of the phase modulation structure on the transmitted pilot signal commonly used channel filtering has not been performed.
Traditional CDMA systems employ “rake” receivers in mobile units and base stations to exploit this path diversity. Rake receivers estimate the timing delay introduced by each of one or more multipaths in comparison with some reference e.g., line-of-sight delay), and then use the estimated timing delays to receive the multipaths that have the highest signal strength. A typical rake receiver includes a number (e.g., three to six) of rake branches or “fingers”. Each finger is an independent receiver unit, which assembles and demodulates one received multipath that is assigned to the finger. A rake receiver also includes a separate “searcher,” which searches out different signal components of an information signal that was transmitted using the assigned signature sequence of the receiver, and detects the phases of the different signal components. The timing of each finger is controlled such that it is correlated with a particular multipath, which arrived at the receiver with a slightly different delay and was found by the searcher. Thus, each finger is “assigned” to a particular multipath by controlling its timing to coincide with arrival of the multipath. The demodulated output from each finger, representing one multipath, is then combined into a high-quality output signal, which combines the energy received from each multipath that was demodulated. The implementation of rake receivers is generally known for both forward and reverse CDMA channels. See, e.g., R. Price and P. E. Green, Jr., A Communication Technique for Multipath Channels, 46 Proc. Inst. Rad. Eng. 555–70 (March 1958); G. Cooper and C. McGillem, Modern Communications and Spread Spectrum, Chapter 12, McGraw-Hill, NY, 1986 incorporated herein for background and, among others, U.S. Pat. Nos. 5,109,390; 6,269,075; and 6,266,365, which patents are incorporated herein for all purposes.
In general, rake receivers estimate the channel using a searcher having a ½ chip resolution (i.e., −0.25/+0.25 chip resolution), and the fingers are assigned using the same resolution. The resolution of the finger assignment creates a timing misalignment between the received signal and the pseudo-noise (PN) sequence generated locally in the finger which results in signal-to-noise ratio (SNR) degradation, or degraded Frame Error Rate (FER) performance. For example, with ½ chip resolution for the searcher and the finger assignment, the resulting timing misalignment of 0.25 chip causes a SNR degradation on the order of 1 dB. Although receivers typically include a delay-locked loop (DLL) to correct such assignment errors, the loss due to the initial timing mis-alignment becomes significant in the dynamic environments faced by CDMA mobile stations, where finger re-assignments may be performed as often as every 5 to 10 frames. The DLL, which typically requires on the order of 2 frames to correct initial timing mis-alignments, is too slow to cause the timing mis-alignment of the initial finger assignment to have a non-negligible effect on receiver performance.
Thus, in existing CDMA systems the fingers of a rake receiver, which are formed by parallel correlators or matched filters, are used to deal with rapid changes in the multipath structure of the received signal, while a plurality of DLLs are used to deal with slight changes in the phase of the spreading waveform caused by the Doppler shift. Rake receivers with multiple DLL's on each fingers perform acceptably when the individual DLL's have to track the phase of a single multipath component. However, their performance degrades sharply when two or more multipath components of about the same amplitude arrive within a chip duration with different Doppler shifts. Since this is the expected case in an urban communication environment, it is a severe limiting factor in the successful deployment of the communication systems.
These above problems are aggravated by the fact that new generation communication systems are aiming at providing enhanced communication service for mobile users, involving higher bit rates communications. For this purpose, standardization efforts for the third generation (3G) communication systems set very demanding communication standards for mobile users. Although there are differences in various standardization approaches, the CDMA-based communication plays a central role in 3G systems. For instance, in the standard for 3G cdma2000 system a CDMA air interface is used to provide wireline-quality voice service and high-speed data services ranging from 144 kilobits per second (Kbps) for mobile users to 2 megabits per second (Mbps) for stationary users. In the 1×EV-DO standard, which is also referred to as the High Data Rate (HDR), the communication rates ranges from 38.4 Kbps to 2457.6 Kbps, respectively. Likewise, in wideband CDMA (W-CDMA) communication standard the communication rates can be as high as 2072 Kbps.
One approach to minimizing the performance problems associated with timing mis-alignments caused by the initial finger assignment in a CDMA system with rake receivers is to use searchers with improved resolution to estimate parameters of the channel. For example, a searcher having ¼ or ⅛ chip resolution could be used. However, the hardware implementation of such high-resolution searchers are more complex than the implementation of ½ chip resolution searchers, and would not be economical or practical for the construction of CDMA mobile stations.
The present invention is directed to an improved system and method in which the multipath parameters are computed in a fast and efficient manner, providing significant improvement over the prior art.
In particular, the 3rd Generation Partnership Project publication mentioned above indicates that the propagation of radio waves in a mobile user environment can be described by multiple paths caused by reflection and scattering of the transmitted signals. As noted, if the user is non-stationary, the signals propagated on each path exhibit a Doppler shift. Since the effective velocity of the mobile user and the base station may differ for each path, the actual Doppler shifts on each path may be different.
The following discussion is illustrated by signal propagation models used in accordance with the present invention. In particular, the received signals at a stationary and a mobile receivers will differ. Thus, if the transmitted signal is denoted as s(t)ej2πƒot, in which s(t) is the information carrying signal modulated on a carrier frequency ƒo, the received signal at a stationary receiver will berRF,stationary,k(t)=Cks(t−τk)ej2πƒot,in which Ck is a complex constant and the delay τk is given by τk=Rk/co in terms of the separation Rk between the transmitter and receiver, and the speed of light co. However, if the receiver is moving at a speed vk away from the transmitter, the received signal will berRF,k(t)=rRF,stationary,k((1−vk/co)t).
An expanded version of the forgoing expression provides a greater insight into the Doppler effect on the transmitted signal s(t),rRF,k(t)=Cks((1−vk/co)t−τk)ej2πƒo(t−νk/cot)=Cks((1−vk/co)t−τk)ej2π(ƒo−νk)t≈Cks(t−τk)ej2π(ƒo−νk)t,  (1)in which the Doppler shift νk=ƒovk/co. From this expression it follows that the Doppler shift is approximately equivalent to a shift in carrier frequency ƒo.
In a multipath channel, the received signal can be approximated as:
                                          r            RF                    ⁡                      (            t            )                          =                              ∑                          k              =              1                        N                    ⁢                                          ⁢                                    C              k                        ⁢                          s              ⁡                              (                                  t                  -                                      τ                    k                                                  )                                      ⁢                                          ⅇ                                                      j2π                    ⁡                                          (                                                                        f                          0                                                -                                                  v                          k                                                                    )                                                        ⁢                  t                                            .                                                          (        2        )            where N is the number of distinct propagation paths each having amplitude Cκ, time delay τκ, and effective Doppler shift νκ, which are also known as propagation parameters. Typically, these propagation parameters change dynamically in time, because individual multipath components arrive at the receiver within a short time, e.g., a few microseconds. For instance, according to the 3rd Generation Partnership Project, in a hilly terrain and urban environments, there may be as many as 20 multipath components that may arrive within about 18.0 and 2.1 microseconds. In practice, the multipath signal components that are reflected in the close proximity of the receiver are detected by the receiver at about the same time, but with highly varying Doppler shifts. Furthermore, for a rapidly moving mobile user, the positions of the reflectors and their corresponding delay and Doppler shifts change considerably over short time intervals. Thus, although over very short time intervals the spectrum of the received signal from the close proximity of the mobile user has distinct spectral components and can be approximated as shown above, the time-averaged spectrum shows continuum-like spread, which is known as the Doppler spread.
Accordingly, the model in Eq. (2) can be used to describe multipath propagation over relatively short time durations. In wireless communication applications, such duration may be about 100 msecs, although it should be apparent the duration can be selected based on the particular application. In accordance with one aspect, the present invention provides systems and methods for the estimation of the multipath propagation parameters that can be conducted over a short time interval and can be refreshed frequently. In accordance with another aspect, for the purpose of estimating the multipath propagation parameters, the present invention uses pilot signals of the base and mobile stations. Since these signals are used in the synchronization of the base and the mobile stations, they are commonly used in the wireless communication systems. The use of pilot signals in wireless communication systems was illustrated in FIGS. 2A, B and 3A, B and are further disclosed, for example, in U.S. Pat. Nos. 5,056,109 and 5,103,459, which are incorporated herein for all purposes. The present invention is based in part of the disclosure of U.S. patent application Ser. No. 09/875116, filed Jun. 6, 2001 to the coinventors of this patents, which application is incorporated herein by reference.
The interested reader is also directed to the disclosure of the following publications, which are incorporated by reference for background. Reference numerals used in the following description correspond to the numbering in the listing below.
[1] 3rd Generation Partnership Project, Technical Specification Group Radio Access Networks, Deployment aspects (Release 4), 3GPP TR 25.943 V4.0.0, June 2001.
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